696 SEXUALITY 



ferent groups. Sonneborn (1938a, 1939a) found two mating types in 

 P. calkinsi and three interbreeding types in P. trichium, indicating a sys- 

 tem of multiple types such as Jennings found in P. bursaria. Giese (1938, 

 1939) and Giese and Arkoosh (1939) have found mating types in P. 

 multimhronucleatum and P. caudatum. 



Kimball (1939c) found in Euplotes, one of the hypotrichous ciliates, 

 a system of mating types like the one in P. bursaria. There are five groups 

 of non-interbreeding types, with morphological differences between some 

 of the groups, indicating that these may be taxonomically as well as 

 genetically different species. In each group occur multiple interbreeding 

 mating types, six in the group most fully studied, any one of these con- 

 jugating with any of the other five. The striking agglutinative mating 

 reaction so characteristic of Paramecium appears to be lacking: conjuga- 

 tion first occurs several hours after mixture of the different types, and 

 then pairs form directly without the prior formation of clusters. 



In view of our present knowledge, it seems allowable to include Mau- 

 pas's (1889) old evidence for the necessity of diverse ancestry as evi- 

 dence for diversity of mating type. If so, at least four more species must 

 be added to the list of those in which mating types are known : Stylony- 

 chia pustulata, Leucophyrs patula, Onychodomus grandis, and Loxophyl- 

 lum fasciola. 



This brings the number of species now known to have mating types 

 to about a dozen. These belong to six different genera and two different 

 orders of ciliate Protozoa. It appears, therefore, that mating types will 

 be found to be widely distributed among the ciliates. The view that any 

 two individuals of the same species can conjugate with each other, if 

 capable of conjugating at all, is demonstrably false; on the contrary, in 

 general, conjugation can take place only between individuals of diverse 

 mating types. 



Are there ever exceptions to this general rule? Does conjugation ever 

 take place between animals of the same mating type? In nearly all the 

 species examined in detail, conjugation has been observed in cultures 

 containing only one caryonide, and, as members of the same caryonide 

 are presumably of the same mating type, this appears to be conjugation 

 between animals of the same mating type. Can individuals of the same 

 caryonide ever differ in mating type? And is this the explanation of these 

 exceptional conjugations within a caryonide? There is only one method 



